1. Field of the Invention
This invention relates to microwaveable thermodynamic receptacles or containers. More specifically, this invention relates to containers adapted to maintain their contents at elevated temperatures for extended periods of time. Thermodynamic containers of this invention include a microwave-absorbing composition in heat exchange relationship with the container contents and preferably in the space defined by the inner and outer walls of the container. Upon exposure to microwave energy, the composition absorbs energy in the form of microwaves, which is retained in the composition in the form of thermal energy. The thermal energy can then be transferred into the contents of the container, such as a food or beverage, thereby maintaining the contents at an elevated temperature for an extended period of time.
2. Background of the Invention and Related Art
Maintaining the temperature of a food or beverage after cooking or preheating has long been of interest in food and beverage service. In the domestic, recreational, field, and commercial environments (among others), maintenance of temperature is desirable when the food or beverage is served after cooking or preheating. In commercial environments, food service pans are often placed over a tray of boiling water or over some other heating means to maintain the temperature of the food after preparation. In the home, it is common for a casserole dish to be placed into a wicker basket or wrapped in a cloth towel to insulate the container and maintain the desired temperature of the contents. For the same purpose, electrically heated trivets, or preheated hot-pads, are sometimes used in the home as well.
It is often desirable to be able to consume a food or beverage, prepared earlier, at some location removed from the home, such as at the workplace. In these circumstances, it is often desirable to place food into a portable container which can be easily carried to a separate location where the food can later be consumed. In some environments, such as in remote wilderness locations, or at some sporting events, it can be impossible or impractical to reheat the food or beverage prior to consumption. In these instances, it is both pleasing and convenient to open the container at some later period and find that the food or beverage has been maintained at a desirable temperature.
Portable containers which serve this purpose have been known for years. Early containers which served this purpose include bottles or other containers insulated with a foam insulation such as foamable foam polystyrene, e.g., STYROFOAM, or foamable foam polyurethane. Although these containers extend the time a food or beverage may be maintained at an elevated temperature, they do not provide the level of temperature maintenance desired in many instances.
Foam insulated beverage containers have also been known for years. A recent advance in double walled foam insulated beverage containers is disclosed in U.S. Pat. No. 5,515,995, to ALLEN et al. This patent discloses a double walled, foam insulated beverage container having a wide base. This patent, and U.S. Pat. No. 3,684,123, to BRIDGES, cited therein, are hereby incorporated by reference as though set forth in full herein.
In general, vacuum insulated bottles are far superior to those insulated with foam. U.S. Pat. No. 3,331,522, to BRIDGES, which is hereby incorporated by reference as though set forth in full herein, describes a vacuum insulated bottle comprising a metal vacuum bottle enclosed in a plastic jacket.
To enhance the ability of the vacuum bottle itself to insulate, attempts were made to utilize different materials for the vacuum bottle. Although fragile, glass is superior to metal in its lack of thermal conductivity, and thus glass vacuum containers became popular for use in thermally insulated containers.
To further enhance the insulating properties of the glass vacuum container, attempts were made to line the glass vacuum containers with reflective coatings to minimize radiant heat loss. U.S. Pat. No. 3,910,441, to BRAMMING, is illustrative. BRAMMING discloses a glass vacuum bottle construction in which the interior walls are silver-coated to reduce heat loss.
With the advent of microwave cooking technology and its rapid acceptance and popularity, the need arose for a thermally insulated bottle which could be used in a microwave oven. However, because metals absorb significant microwave energy, they can become dangerously hot in a microwave oven. In addition, metals and coatings containing metals, such as silver coatings, can damage the microwave oven magnatron tube by reflecting microwave energy back into the wave guide, and by "arcing" and/or sparking.
U.S. Pat. No. 4,184,601, to STEWART et al., which is hereby incorporated by reference as though set forth in full herein, relates to a microwave-safe vacuum insulated container. The STEWART container utilizes a glass vacuum container. However, instead of a silver lining to reduce radiant heat loss, the annular space of the glass container is substantially filled with finely divided materials which are neither electrically conductive nor absorbent of electromagnetic energy at microwave frequencies. Examples of such materials include finely divided silica and calcium carbonate.
While the vacuum containers which are known in the art are certainly able to conserve the heat of their contents, a continuing need for improvement remains. Most commercially-available vacuum containers known in the art allow a significant loss of thermal energy at a measurable rate. The ability of the container to maintain the heat of its contents is measured by the rate of thermal energy loss (heat loss/unit of time), which can be considered its thermal efficiency. A need exists for improved thermal efficiency in a microwaveable container.
The present invention enhances the ability of the thermally insulated container to maintain contents at elevated temperatures. Vacuum containers are directed to conserving the thermal energy already present in the contents of the container. The present invention transfers thermal energy into the contents, adding to the thermal energy of the contents, and thereby keeping the contents at a higher temperature for extended periods of time.
The concept of a container which can be preheated to add thermal energy to the contents is not new. For example, U.S. Pat. No. 4,567,877, to SEPAHPUR, discloses a heat storage food container, adapted to be used in microwave ovens. SEPAHPUR uses wet sand as the thermal storage medium. However, an obvious drawback to the SEPAHPUR container is that water undergoes a phase transition (from liquid to gas) upon heating in the range necessary for food preparation. Upon the phase change from liquid to gas, the molecules occupy a significantly greater volume. Consequently, the heat storage container must be engineered to structurally accommodate such changes.
U.S. Pat. No. 5,052,369, to JOHNSON, which is hereby incorporated by reference as though set forth in full herein, also discloses a heat retaining food container adapted for microwave use. Unlike SEPAHPUR, the microwave absorbing material of JOHNSON is a mixture of micro crystalline wax which exhibits a fusion temperature (melting point) between 175.degree. F. and 300.degree. F. However, these temperatures are well above those at which food or beverages are often consumed.
In addition to wax, other heat storage materials have been employed as well. For example, U.S. Pat. No. 4,983,798, to ECKLER et al., discloses the use of materials which undergo a "mesocrystalline" phase change in the solid state prior to melting, such as pentaerythritol and neopentyiglycol. U.S. Pat. No. 5,424,519, to SALEE, discloses the use of "wax in liquid" or "oil in liquid" emulsions to act as a heat storage devices. This patent discloses that these materials may be heated by microwave energy to, e.g., 30.degree.-90.degree. C. by exposing to microwave energy at 700 to 750 watts for approximately 3 to 15 minutes. U.S. Pat. No. 5,282,994, to SALYER, discloses a heat storage composition comprising a phase change material which can include polar materials such as glycerine, ethylene glycol, and polyethylene glycol. Each of these patents is hereby incorporated by reference as though set forth in full herein.
Commercially available standard microwave ovens for domestic use in the home typically are rated as having an "output" of, for example, on the order of from about 600 to about 1,000 watts. Typical commercial "convenience" foods are specifically designed to be heated to or near a desired or serving temperature (e.g., perceived to be desirable by the typical adult) in from about 2 to about 6 minutes. Consequently, it is believed that typical users of microwave ovens in domestic settings desire or expect to employ a microwave oven to heat the contents of a container to a desired serving temperature in a period of time of from about 2 to about 6 minutes.